We have conducted immunocytochemical studies on two glycoproteins on the surfaces of colon epithelial cells. Carcinoembryonic antigen (CEA) and secretory component (SC) were localized by peroxidase-labeled antibodies at the light microscopic levels. In normal colonic epithelium, CEA was identified prominently on the microvillar surfaces; very little was found on the basolateral surface. SC was restricted to the basolateral surface. This highly segregated distribution of the two glycoproteins was seen also a well-differentiated colon cancer and in HT-29 cultured colon cancer cells. However, in poorly differentiated cancer cells, little SC was found, and CEA was distributed over the entire cell surface. We conclude that regional segregation of certain surface glycoproteins is a characteristic of well-differentiated, polarized epithelial cells; this characteristic is absent from malignant cells when undifferentiated. In other work, we studied IgA in human bile and liver. IgA was found to comprise about 40% of bile proteins; the biliary IgA was characterized as dimeric and associated with SC. SC and IgA were demonstrated immunocytochemically in intra-and extrahepatic biliary epithelium. We conclude that IgA, a prominent protein in bile, is translocated across biliary epithelium by an SC-mediated mechanism.